U.S. patent number 8,884,525 [Application Number 13/425,159] was granted by the patent office on 2014-11-11 for remote plasma source generating a disc-shaped plasma.
This patent grant is currently assigned to Advanced Energy Industries, Inc.. The grantee listed for this patent is Daniel Carter, Randy Grilley, Daniel J. Hoffman, Karen Peterson. Invention is credited to Daniel Carter, Randy Grilley, Daniel J. Hoffman, Karen Peterson.
United States Patent |
8,884,525 |
Hoffman , et al. |
November 11, 2014 |
Remote plasma source generating a disc-shaped plasma
Abstract
Disclosed herein are systems, methods and apparatuses for
dissociating a non-activated gas through a disc-shaped plasma in a
remote plasma source. Two inductive elements, one on either side of
the disc-shaped plasma, generate a magnetic field that induces
electric fields that sustain the disc-shaped plasma. The inductive
elements can be coiled conductors having any number of loops and
can be arranged in planar or vertical coils or a combination of
planar and vertical coils. Additionally, the ratio of inductive
element radius to gap distance between the two inductive elements
can be configured to achieve a desired vertical plasma
confinement.
Inventors: |
Hoffman; Daniel J. (Fort
Collins, CO), Carter; Daniel (Fort Collins, CO), Grilley;
Randy (Ault, CO), Peterson; Karen (Loveland, CO) |
Applicant: |
Name |
City |
State |
Country |
Type |
Hoffman; Daniel J.
Carter; Daniel
Grilley; Randy
Peterson; Karen |
Fort Collins
Fort Collins
Ault
Loveland |
CO
CO
CO
CO |
US
US
US
US |
|
|
Assignee: |
Advanced Energy Industries,
Inc. (Fort Collins, CO)
|
Family
ID: |
46876774 |
Appl.
No.: |
13/425,159 |
Filed: |
March 20, 2012 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20120242229 A1 |
Sep 27, 2012 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61466024 |
Mar 22, 2011 |
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Current U.S.
Class: |
315/111.21;
219/121.41; 219/121.4; 315/111.71; 315/111.91; 315/111.41;
315/111.51 |
Current CPC
Class: |
H05H
1/46 (20130101); H05H 1/4652 (20210501) |
Current International
Class: |
H05B
31/26 (20060101); B23K 10/00 (20060101) |
Field of
Search: |
;315/111.21,111.41,111.51,111.71,111.91
;219/121.4,121.41,121.43,121.52,121.54,121.57 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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04-193329 |
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Jul 1992 |
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JP |
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2012103101 |
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Aug 2012 |
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WO |
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Primary Examiner: Tan; Vibol
Attorney, Agent or Firm: Neugeboren O'Dowd PC
Claims
What is claimed is:
1. A remote plasma source comprising: a first inductive coil having
a first plurality of loops, the first plurality of loops having an
average radius R1; a second inductive coil having a second
plurality of loops, the second plurality of loops having the
average radius R1, wherein the first and second inductive coils are
parallel to each other and separated by a distance D, wherein the
first and second inductive coils are configured to conduct an
alternating current to generate magnetic fields that sustain a
disc-shaped plasma between the first and second inductive coils,
wherein the alternating current sustains the disc-shaped plasma
primarily through inductive coupling; a chamber disposed between
the first and second inductive coils, and configured to enclose the
disc-shaped plasma; a first dielectric layer parallel to the first
and second inductive coils and disposed between the chamber and the
first inductive coil, wherein the first dielectric layer is
configured to reduce capacitive coupling between the first
inductive coil and the disc-shaped plasma and allow the magnetic
fields to pass from the first inductive coil to the disc-shaped
plasma; and a second dielectric layer parallel to the first and
second inductive coils and arranged between the chamber and the
second inductive coil, wherein the second dielectric layer is
configured to reduce capacitive coupling between the second
inductive coil and the disc-shaped plasma and allow the magnetic
fields to pass from the second inductive coil to the disc-shaped
plasma; a gas entry connected to the chamber and configured to
provide non-activated gas to the chamber; and a gas exit connected
to the chamber and configured to enable activated gas and free
radicals to exit the chamber.
2. The system of claim 1, wherein the first and second inductive
coils are solenoid-shaped inductors.
3. The system of claim 1, wherein the first and second inductive
coils are planar inductors.
4. The system of claim 1, wherein the first and second inductive
coils comprise two or more windings stacked vertically like a
solenoid and two or more windings arranged in a planar
dimension.
5. The system of claim 1, wherein the gas entry is arranged to
provide the non-activated gas in a direction parallel to the first
and second inductive coils and intersecting a portion of the
disc-shaped plasma.
6. The system of claim 1, wherein the disc-shaped plasma has a
plasma density that increases towards a center of the chamber.
7. A method comprising: providing a reactive gas to a remote plasma
source chamber through a gas entry connected to the chamber that is
configured to provide reactive gas to the chamber; passing a high
voltage current through a first inductor and a second inductor to
generate an electric field passing from the first inductor through
a first dielectric layer, through the remote plasma source chamber,
through a second dielectric layer, and to the second inductor,
wherein the electric field is strong enough to ignite a plasma in
the reactive gas in the remote plasma source chamber; passing an
alternating current through the first inductor and the second
inductor to inductively induce mirror electric fields in the
plasma, wherein the induced mirror electric fields propagate in an
opposite direction to the alternating current, and wherein the
induced mirror electric fields sustain the plasma; and dissociating
the reactive gas by passing it through the plasma to form activated
gas and free radicals; and removing the activated gas and free
radicals from the remote plasma source chamber through a gas exit
connected to the chamber that is configured to enable the activated
gas and free radicals to exit through the chamber.
8. The method of claim 7, further comprising directing an
alternating magnetic field between the first and second inductors
in a direction perpendicular to a first inner surface and a second
inner surface of the remote plasma chamber.
9. The method of claim 7, wherein the alternating magnetic field
has an equivalent field density at the first and second inner
surfaces of the remote plasma chamber.
10. A system comprising: a remote plasma source chamber having
parallel first and second surfaces; a first coiled conductor
arranged outside the remote plasma source chamber and adjacent to
the first surface of the remote plasma source chamber, wherein the
first coiled conductor generates a first magnetic field directed
into the remote plasma source chamber and primarily in a first
direction perpendicular to the first and second surfaces; a first
dielectric arranged between the first surface and the first coiled
conductor; a second coiled conductor arranged outside the remote
plasma source chamber and adjacent to the second surface of the
remote plasma source chamber, wherein the second coiled conductor
generates a second magnetic field primarily in the first direction;
a second dielectric arranged between the second surface and the
second coiled conductor; a reactive gas entry that directs a
reactive gas into the remote plasma source chamber in a second
direction tangential to an outermost portion of the first coiled
conductor and perpendicular to the first direction; and a radicals
exit port that removes radicals formed when the reactive gas is
passed through a plasma disc formed in the remote plasma source
chamber.
Description
FIELD OF THE INVENTION
The present invention relates generally to plasma processing. In
particular, but not by way of limitation, the present invention
relates to systems, methods and apparatuses for dissociating a
reactive gas into radicals.
BACKGROUND OF THE INVENTION
Passing a gas through a plasma can excite the gas and produce
activated gases containing ions, free radicals, atoms and
molecules. Activated gases and free radicals are used for numerous
industrial and scientific applications including processing solid
materials such as semiconductor wafers, powders, and other gases.
Free radicals are also used to remove deposited thin films from
semiconductor processing chamber walls.
Where activated gases or free radicals are used in processing, it
may be desirable to preclude the plasma from interacting with the
processing chamber or semiconductors being processed. Remote plasma
sources can fill this need by generating the plasma, activated
gases, and/or free radicals in a chamber that is isolated from the
processing chamber, and then passing only the activated gases
and/or free radicals to the processing chamber.
Plasmas can be generated in various ways, including DC discharge,
radio frequency (RF) discharge, and microwave discharge. DC
discharges are achieved by applying a potential between two
electrodes in a gas. Plasmas generated via RF and DC currents can
produce high-energy ions able to etch or remove polymers,
semiconductors, oxides, and even metals. Therefore, RF or
DC-generated plasmas are often in direct contact with the material
being processed. Microwave discharges produce dense, low ion energy
plasmas and, therefore, are often used to produce streams of
activated gas for "downstream" processing. Microwave discharges are
also useful for applications where it is desirable to generate ions
at low energy and then accelerate the ions to the process surface
with an applied potential.
Existing remote sources (e.g., toroidal and linear remote sources)
have four main drawbacks. First, they fail to pull the plasma away
from the remote source chamber walls thus allowing the plasma to
etch the chamber walls. This will be referred to as poor plasma
confinement. Second, they use a high power density to sustain the
plasma, which generates high energy ions that bombard the remote
source chamber walls and the processing chamber walls. Ion
bombardment can also damage the wafers or other semiconductors
being processed in the process chamber (e.g., etching low-k
dielectrics). Third, toroidal and linear remote sources have
significant electrostatic coupling to the plasma, which leads to
further ion bombardment. Finally, these sources provide a narrow
plasma cross-section through which non-activated or non-ionized gas
can pass through. Thus, they may be limited in their effectiveness
at dissociating non-activated gas.
SUMMARY
Illustrative embodiments of the present disclosure are shown in the
drawings and summarized below. These and other embodiments are more
fully described in the Detailed Description section. It is to be
understood, however, that there is no intention to limit the claims
herein to the forms described in this Summary or in the Detailed
Description. One skilled in the art can recognize that there are
numerous modifications, equivalents, and alternative constructions
that fall within the spirit and scope of the present disclosure as
expressed in the claims.
In one embodiment, the invention may be characterized as a remote
plasma source. In this embodiment, the remote plasma source
includes a first inductive coil having a first plurality of loops
and a second inductive coil having a second plurality of loops,
wherein the first and second inductive coils are parallel to each
other. The first and second inductive coils are configured to
conduct an alternating current to generate magnetic fields that
sustain a disc-shaped plasma between the first and second inductive
coils, wherein the alternating current sustains the disc-shaped
plasma primarily through inductive coupling. And a chamber disposed
between the first and second inductive coils, and configured to
enclose the disc-shaped plasma.
Another aspect of the invention may be characterized as a method
for providing a reactive gas to a remote plasma source chamber. The
method includes passing a high voltage current through a first
inductor and a second inductor to generate an electric field
passing from the first inductor through the remote plasma source
chamber and to the second inductor wherein the electric field is
strong enough to ignite a plasma in the reactive gas in the remote
plasma source chamber. In addition, an alternating current is
passed through the first inductor and the second inductor to
inductively induce minor electric fields in the plasma. The
reactive gas is dissociated by passing it through the plasma to
form activated gas and free radicals, and the activated gas and
free radicals are removed from the remote plasma source
chamber.
Another aspect of the invention may be characterized as a system
that includes a remote plasma source chamber having parallel first
and second surfaces, a first coiled conductor arranged outside the
remote plasma source chamber and adjacent to the first surface of
the remote plasma source chamber, a first dielectric arranged
between the first surface and the first coiled conductor, a second
coiled conductor arranged outside the remote plasma source chamber
and adjacent to the second surface of the remote plasma source
chamber, and a second dielectric arranged between the second
surface and the second coiled conductor. In addition, a reactive
gas entry directs a reactive gas into the remote plasma source
chamber and a radicals exit port removes radicals formed when the
reactive gas is passed through the plasma disc formed in the remote
plasma source chamber.
BRIEF DESCRIPTION OF THE DRAWINGS
Various objects and advantages and a more complete understanding of
the present invention are apparent and more readily appreciated by
reference to the following Detailed Description and to the appended
claims when taken in conjunction with the accompanying Drawings
where like or similar elements are designated with identical
reference numerals throughout the several views and wherein:
FIG. 1 illustrates a profile view of an embodiment of an exemplary
remote plasma source.
FIG. 2 illustrates a profile view of an embodiment of a remote
plasma source as described in this disclosure.
FIG. 3A illustrates a profile view of an embodiment of a remote
plasma source showing magnetic field lines generated by the
conductors.
FIG. 3B illustrates a profile view of an embodiment of a remote
plasma source showing electric field lines in a plasma that are
induced by the magnetic field illustrated in FIG. 3A.
FIG. 4 illustrates a profile view of an embodiment of a remote
plasma source having conductors arranged in two radial coils.
FIG. 5 illustrates an overhead view of an embodiment of a remote
plasma source having a circular first conductor connected to an AC
source.
FIG. 6 illustrates a profile view of an embodiment of a remote
plasma source having conductors arranged in two vertical coils.
FIG. 7 illustrates a profile view of an embodiment of a remote
plasma source having conductors arranged in a radial and vertical
configuration.
DETAILED DESCRIPTION
Applicants have found that the deficiencies of existing remote
sources (e.g., toroidal and linear remote sources) can be solved
via a remote plasma source having two circular or coiled
conductors. The use of two conductors with mirrored AC passing
through them achieves far greater plasma confinement and lower
plasma densities than the prior art. This is in part due to the
creation of a disc-shaped plasma rather than a toroidal or tubular
plasma as seen in the prior art. Additionally, the disc-shaped
plasma presents a greater cross section through which non-activated
gas can be passed. The two circular or coiled conductors can be
spaced from each other and have a radius per winding that falls
within a range of values that allow the plasma to be sustained with
low power density, low electrostatic coupling, and that will
confine the plasma to a much greater extent than the prior art.
FIG. 1 illustrates a profile view of an embodiment of a remote
plasma source as described in this disclosure. The remote plasma
source 300 includes a remote plasma source chamber 302 that
encloses a volume 320 in which the plasma 342 is confined. As
shown, the volume 320 in this embodiment is bounded by a first
inner surface 316, a second inner surface 318, and a third inner
surface 324. In addition, the remote plasma source 300 includes a
first inductive element 304 and a second inductive element 306.
When AC current is passed through the first and second inductive
elements 304, 306 an alternating magnetic field 350 passes in the
vertical direction (parallel to the axis 370) between the first and
second inductive elements 304, 306. The alternating magnetic field
350 induces electrical fields that circulate around axis 370 and
induce currents in the plasma 342 that sustain the plasma 342. The
remote plasma source 300 includes a gas entry 308 and a gas exit
310 for providing non-activated gas to the remote plasma source
chamber 302 and for removing activated gas and free radicals from
the remote plasma source chamber 302, respectively.
Although a single inductive element, 304 or 306 could be used to
sustain the plasma 342, vertical containment would be poor because
a single inductive element would cause the plasma 342 to have a
high density near the first or second inner surface 316, 318,
depending on whether the first inductive element 304 or the second
inductive element 306 is used. This high plasma density near either
surface 316, 318 would cause undesired etching of the inside of the
remote plasma source chamber 302; thus to pull the plasma 342 off
of one of the walls, two inductive elements 304, 306 in the
exemplary embodiment are used. In this way, the plasma 342 is
vertically contained away from both of the inner surfaces 316, 318
to an extent previously unseen.
In addition, vertical confinement may be further enhanced by
selecting certain ratios of the radii of the inductive elements
304, 306 versus a distance between the inductive elements 304, 306.
For particular ratios, a potential energy of the plasma 342 is such
that the plasma 342 is further confined to a center of the volume
320. For instance, a nitrogen plasma density in the 10.sup.11 to
10.sup.12 cm.sup.-3 range can be pulled off the walls for the dual
coils configured to produce .about.7 Gauss rms at the center of the
plasma.
FIG. 2 illustrates a profile view of another embodiment of a remote
plasma source 400. The remote plasma source 400 includes a remote
plasma source chamber 402 in which a plasma 442 is confined. As
depicted the chamber includes a volume 420 that is bounded by a
first inner surface 416, a second inner surface 418, and a third
inner surface 424. The remote plasma source 400 includes a first
and second conductor 404, 406, and in the illustrated embodiment,
current in the conductors 404, 406 directed into the page is
indicated by a circle enclosing an "x" and current directed out of
the page is indicated by a circle enclosing a dot. These currents
generate the induced electric fields 430 in the plasma 442, which
in turn generate image currents at the same location as the induced
electric fields 430. As shown, the remote plasma source 400
includes a first dielectric 412 arranged between the first
conductor 404 and the remote plasma source chamber 402 and a second
dielectric 414 is arranged between the second conductor 406 and the
remote plasma source chamber 402. The remote plasma source 400
includes a gas entry 408 and a gas exit 410.
The remote plasma source chamber 402 can be made of a ceramic or
any other material that allows passage of a magnetic field
generated by the conductors 404, 406. The remote plasma source
chamber 402 can be shaped like a cylinder (viewed here in profile).
And from above, the remote plasma source chamber 402 appears as a
circle. And the first and second inner surfaces 416, 418 can be
parallel to each other and perpendicular to an axis 470. The third
inner surface 424 can be perpendicular to the first and second
inner surfaces 416, 418, and parallel to and radially disposed
around the axis 470. In this embodiment, the axis 470 passes
through a middle or center of the remote plasma source chamber 402
such that the third inner surface 424 is always equidistant from
the axis 470.
As depicted, the dielectrics 412, 414 can touch an outer surface of
the remote plasma source chamber 402 and can be separated by
corresponding air gaps from the conductors 404, 406. The air gaps
along with the dielectrics 412, 414 impede electric fields
generated by the conductors 404, 406 directed towards the plasma
442. As such, the dielectrics 412, 414 and the air gap decrease
electrostatic coupling between the conductors 404, 406 and the
plasma 442. In one variation of the present embodiment, a faraday
shield can be arranged between the dielectrics 412, 414 and the
conductors 404, 406 to further reduce electrostatic coupling to the
plasma 442. In another variation of the present embodiment, the
dielectrics 412, 414 can touch the conductors 404, 406.
The gas entry 408 can be configured to provide non-activated gas to
the volume 420. The gas entry 408 can be arranged to be flush with
the third inner surface 424 such that the gas entry 408 does not
protrude into the volume 420. In such an embodiment, the
non-activated gas enters the volume 420 at a radius from the axis
470 equal to the radius of the third inner surface 424. In an
alternative embodiment, the gas entry 408 can be arranged within
the volume 420 such that the non-activated gas enters the volume
420 at a radius less than the radius of the third inner surface
424. For instance, the gas entry 408 can be arranged to release
non-activated gas into the volume 420 at a radius equal to the
radius from the axis 470 of the conductors 404, 406. The gas entry
408 can be arranged at an angle and radius from the axis 470 that
enables the non-activated gas to be released into the volume 420 at
a point and direction tangential to, or near tangential to the
plasma 442.
The gas entry 408 can also be positioned and directed to release
gas tangential to the electric fields. For example, the gas entry
408 can be arranged at a position and angle tangential to the
conductors 404, 406. In other words, assuming an imaginary cylinder
is formed that passes through both conductors 404, 406, the gas
entry 408 can be aligned tangential to the imaginary cylinder. In
terms of vertical orientation, the gas entry 408 can be arranged
midway between the first and second conductors 404, 406. The gas
entry 408 can release non-activated gas in a direction parallel to
the conductors 404, 406.
In contrast to typical linear remote plasma sources, which release
and flow non-activated gas in a direction parallel with the
respective magnetic fields, the non-activated gas in the present
embodiment can be released into the volume 420 in a direction
perpendicular to the vertical magnetic fields generated by the
conductors 404, 406.
The gas exit 410 can be configured to remove or allow the release
of activated gas and free radicals from the volume 420. A lifetime
of the plasma's 442 prevents it from diffusing through or being
pulled through the gas exit 410 before the plasma is extinguished.
The gas exit 410 can be arranged flush with the third inner surface
424 and can provide a path for activated gas and free radicals to
be transported to a processing chamber (not illustrated).
The first and second conductors 404, 406 can be parallel to each
other, and they can have a circular or coiled shape. In the
illustrated embodiment, the conductors 404, 406 have a circular
shape with a constant radius. This can be referred to as a
single-loop or single-winding embodiment. However, it is to be
understood that the conductors 404, 406 can also be coiled in a
spiral formation, and thus have a varying radius. In the
illustrated embodiment, the radius of the outermost portion of the
conductors 404, 406 is less than the radius of the third inner
surface 424. This prevents plasma from being sustained too close to
the third inner surface 424 and thus helps ensure radial plasma
confinement.
How far, in terms of the radial distance from the axis 470, the
third inner surface 424 is located from the conductors 404, 406
accounts for inherent plasma expansion. More specifically, the
magnetic field causes the plasma to have a radial force pushing it
outwards towards the third inner surface 424, but the plasma does
not reach the third inner surface 424 because it is extinguished as
it moves away from the induced electric fields 430. As such, when
the conductors 404, 406 are arranged at least a minimum distance
inside the radius of the third inner surface 424, the plasma is
self-containing in the radial directions. Thus, etching of the
third inner surface 424 can be avoided.
Each conductor 404, 406 can be connected to an alternating current
source such that the polarity, amplitude, and phase in each
conductor 404, 406 are equal. Multiple current sources can also be
used. The voltage from one end of each conductor 404, 406 to
another end of each conductor 404, 406 is highly flexible. For
instance, the conductors 404, 406 can each have a potential
difference of 1 V, but the high and low potential can be +0.25 V
and -0.75 V. As another example, the potential difference could be
1 V, but the high and low potential can be 0 V and 1.0V. Numerous
other combinations are also possible.
In other embodiments, the conductors 404, 406 can be arranged
radially (see for example, FIG. 4), vertically (see for example,
FIG. 6), or in a combination of radial and vertical geometries (see
for example, FIG. 7). And the first conductor 404 can have a
current direction opposite to that in the second conductor 406.
FIG. 3A illustrates a profile view of an embodiment of a remote
plasma source 500 showing magnetic field lines generated by the
conductors. In the illustrated embodiment, a magnetic field 550 is
directed from the first conductor 504 towards the second conductor
506. When the AC current generating the magnetic field 550 flips
polarity, the magnetic field 550 is directed from the second
conductor 506 towards the first conductor 504. In other words, the
direction of current in the conductors 504, 506 determines the
direction of the magnetic field 550. Between the conductors 504,
506 in the vertical dimension, the magnetic field 550 partially
leaks out past a radius of the conductors 504, 506. The result is
that the magnetic field 550 strength within the volume 520 has a
profile resembling a curved hour glass--the magnetic field 550 is
strongest closest to the first and second inner surfaces 516, 518
and weakest halfway between the conductors 504, 506. But magnetic
field 550 strength in the radial direction is greatest close to the
axis 570 and gets weaker moving away from the axis 570 and towards
the third inner surface 524. This magnetic field 550 induces
electric fields that circle the axis 570 in a direction opposite to
that of the currents in the conductors 504, 506.
FIG. 3B illustrates a profile view of an embodiment of the remote
plasma source 500 showing electric field lines in a plasma that are
induced by the magnetic field illustrated in FIG. 3A. Since the
magnetic field lines are directed downwards in the illustrated
embodiment, the induced electric field lines 530 (FIG. 3B) go into
the page on the right and out of the page on the left. This is the
opposite direction to the currents in the conductors 504, 506. In
other words the induced electric fields 530 image the currents in
the conductors 504, 506. These induced electric fields 530 in turn
push a current in the plasma 542 in the same direction as the
induced electric fields 530. Thus, the induced electric fields 530
symbols in FIG. 3B overlap with the symbols for the induced
current. Hereinafter, terminology for the induced electric fields
530 and the induced current will be used interchangeably.
The induced electric fields 530 in this embodiment ionize
non-activated gas that is introduced into the volume 520 and
sustain the plasma 542. The plasma 542 tends to have a profile that
matches that of the induced electric fields 530. However, the
plasma profile can be larger than the induced electric fields 530
profile due to plasma diffusion. In other words, while the induced
electric fields 530 ionize the non-activated gas and generate the
plasma 542, some of the plasma 542 spreads out or diffuses from
ionization locals.
This diffusion is responsible for one of two types of plasma
confinement that embodiments described herein enable. The first
type of plasma confinement is radial--the forces and circumstances
that minimize the amount of plasma 542 that contacts the third
inner surface 524. The second type of plasma confinement is
vertical--the forces and circumstances that minimize the amount of
plasma 542 that contacts the first and second inner surfaces 516,
518.
Radial confinement is an issue since magnetic fields in the plasma
542 create radially-expansive forces on the plasma 542. Without a
countervailing force, the plasma 542 would substantially contact
the third inner surface 524 and etch it. But because plasma cannot
exist long without being sustained by the induced electric fields
530, the plasma 542 is extinguished as it diffuses and expands
radially away from the induced electric fields 530. As a
consequence, although there is a force pushing the plasma 542 to
expand radially towards the third inner surface 524, the plasma 542
is extinguished before it reaches the third inner surface 524.
Thus, as long as the conductors 504, 506 are located at a radius
that is not too close to the radius of the third inner surface 524,
the plasma can be considered radially confined and will not
substantially etch the third inner surface 524.
Vertical confinement prevents the plasma 542 from substantially
contacting the first and second inner surfaces 516, 518. This
confinement is due to two effects: (1) vertical smearing of the
plasma and thus decreased plasma density due to the use of two
conductors 504, 506 rather than just one conductor; and (2) an
optimized conductor 504, 506 loop radius R versus a conductor-gap
distance D that creates a situation where plasma potential energy
is minimized midway between the conductors 504, 506.
Vertical smearing of the plasma results from the use of the two
conductors 504, 506 arranged on opposite sides of the plasma 542.
Recall from FIG. 3A that the magnetic field 550 strength is
strongest near the first and second inner surfaces 516, 518. If
there were only one conductor, then the magnetic field strength
would be strongest near the inner surface closest to the conductor.
In that case, the plasma density would be greatest against that
inner surface and gradually decrease the further from the first
inner surface the plasma gets. The plasma would thus be sucked up
against the first inner surface and etch it. This is essentially
what happens in known inductive single-coil non-remote plasma
sources.
In order to better confine the plasma 542 and pull it off the first
inner surface 516, the second conductor 506 is added. Now, the
magnetic field 550 strength is strongest near the first and second
inner surfaces 516, 518. Instead of the bulk of the magnetic field
550 strength existing near the first inner surface 516, the
magnetic field 550 is smeared in the vertical dimension such that
it bunches up against both the first and second inner surfaces 516,
518. The effect of using two conductors 504, 506 is thus to lower
the magnetic field 550 strength near both of the inner surfaces
516, 518 as compared to the situation where either conductor 504,
506 was used by itself. Since the magnetic field 550 strength is
reduced, the induced currents at the induced electric fields 530,
and thus plasma 542 density, are also reduced. So, although the
plasma 542 is still expected to contact the first and second inner
surfaces 516, 518, the plasma 542 density making contact is
expected to be much less than if only a single conductor 504, 506
is used. In other words, the plasma 542 is smeared in the vertical
direction (e.g., it has a smaller density gradient) when two
conductors 504, 506 are used instead of just one. Thus, the use of
the two conductors 504, 506 advantageously decreases the plasma 542
density near the first and second inner surfaces 516, 518 to assist
in vertical confinement.
But Applicants discovered that vertical confinement is even better
than predicted. The added confinement is unexpectedly due to
minimized plasma 542 potential in the middle of the volume 520
halfway between the first and second inner surfaces 516, 518. As
noted above, one would expect the plasma 542 to have the greatest
density near the first and second inner surfaces 516, 518. Yet, as
seen in FIG. 5B, this expectation does not manifest itself in
practice. Rather, the induced electric fields 530 are strongest
near the midpoint between the conductors 504, 506--where the
magnetic field 550 is weakest. This unexpected result can be
explained by looking at the potential energy of the plasma.
Normally an induced current in a plasma images the conductor that
induced the magnetic field that is responsible for the induced
current. However, when a second conductor is used, the induced
current images two conductors and can do so with the least amount
of energy when the induced current resides at a midpoint between
the two conductors. Hence, the vertical confinement of the induced
electric fields 530 and the plasma 542.
Vertical confinement can be optimized via a unique
frequency-dependent relationship between a radius R of the
conductors 504, 506 and a distance D between the conductors. The
radius R is measured from the axis 570 to an inside edge of the
conductors 504, 506. Frequency-dependent means that the optimum
relation between R and D depends on the AC frequency in the
conductors 504, 506.
The currents induced by the induced electric fields 530 also induce
magnetic fields (not illustrated) that circle the induced electric
fields 530. As the distance D gets smaller (i.e., the first and
second conductors 504, 506 are moved closer to each other), these
induced magnetic fields can gradually start to cancel the magnetic
field 550. At a certain distance D, the induced magnetic fields
cancel the magnetic field 550.
In other embodiments, the conductors 504, 506 can be arranged
radially (see FIG. 4), vertically (see FIG. 6), or in a combination
of radial and vertical geometries (see FIG. 7). In each of these
configurations, the single-loop configuration illustrated in FIG. 2
with physics as described with reference to FIGS. 3A and 3B,
roughly approximates a single loop of these coiled configurations,
which is helpful to provide an understanding of the spiral-type,
multiple-loop embodiments described further herein in connection
with FIGS. 4, 6 and 7. For example, the physics behind the
embodiments in FIGS. 4, 6 and 7, may be better understood by
considering the superposition of multiple loops (such as the loops
described with reference to FIGS. 3A and 3B) that each have a
different radius R.
FIG. 4 illustrates a profile view of an embodiment of a remote
plasma source depicting a cross-section of conductors that are
arranged in two radial coils. When viewed from above, the
conductors 604, 606 have a spiral shape, and when viewed in
profile, as in FIG. 4, the conductors 604, 606 are planar--they are
parallel to the first and second inner surfaces 616, 618. In this
embodiment, current in the conductors 604, 606 can be passed from
the outermost loops towards the innermost loops or vice versa. The
induced currents 630 in the plasma 642 image the currents in the
conductors 604, 606. When the radius of the innermost loops are
close enough together, as for example in the illustrated
embodiment, the plasma 642 forms a disc that is filled with plasma
near the axis 670. In other words, there is no absence of plasma at
the axis. But in other embodiments, the innermost loops do not have
to be so close together. For example, the innermost loops can have
a radius such that plasma is substantially absent near the axis 670
so that the plasma disc can be shaped like a washer.
As compared to the single-loop embodiment described with reference
to FIG. 2, this embodiment can generate a plasma disc having a much
greater cross section for the non-activated gas to pass through. As
a consequence, greater dissociation of the non-activated gas is
achieved with this embodiment. At the same time, the radial remote
plasma source 600 can generate a larger volume of plasma 642, but
use the same power input as the single-loop embodiment of FIG. 2.
The plasma 642 therefore has a lower power density than in the
single-loop embodiment, and a lower power density means fewer
highly-charged ions bombarding the inner surfaces 616, 618, 624 of
the remote plasma source chamber 602. Spreading the plasma 642
radially also means that the surface area where plasma 642 contacts
the first and second inner surfaces 616, 618 is greater than in the
single-loop embodiment. Spreading the same plasma over a larger
surface area results in less plasma density and thus less etching
of the first and second inner surfaces 616, 618.
The gas entry 608 can be arranged at a position and angle
tangential to the outermost conductors. In other words, assuming an
imaginary cylinder passing through both outermost conductors, the
gas entry 608 can be aligned tangential to the imaginary cylinder.
Gas entry 608 can release non-activated gas into the volume 620
parallel to the conductors 604, 606 and at any angle between
tangential to the plasma 642 and directed at the axis 670. In other
words, the non-activated gas can be directed at any point on the
plasma 642 disc, but preferably not directed at the axis 670. This
helps to establish a circulating gas and plasma 642 flow.
In the depicted embodiment, the plasma can be electrostatically
ignited. For example, before any plasma exists in the volume 620,
an electric potential can be formed between the first and second
conductors 604, 606. This potential creates an electric field
through the volume 620. When the field is strong enough it begins
to ionize atoms and break apart molecules. Each ionized atom and
ripped-apart molecule shoots off electrons and other particles that
further ionize surrounding atoms and split surrounding molecules.
Ignition is thus a run-away process that feeds off itself until the
non-activated gas in the volume 620 is largely converted to the
plasma 642.
FIG. 5 illustrates an overhead view of an embodiment of a remote
plasma source having a circular first conductor connected to an AC
source. The chamber 702 resides between the first conductor 704 and
the second conductor (not visible). The first conductor 704 and
second conductors are biased by an AC source 770. For the purposes
of this illustration, only the first conductor 704 will be
described, but it is to be understood that all descriptions of the
first conductor 704 also apply to the non-visible second
conductor.
The AC source 770 can pass AC current through any portion of the
first conductor 704. For instance, in the illustrated embodiment,
AC current passes through the entire first conductor 704. In
another embodiment, the AC source 770 can be connected to the first
conductor 704 such that AC current only passes through 90% of the
first conductor 704, for example. That portion of the first
conductor 704 that current does not pass through can be at the same
potential as a closest point on the first conductor 704 through
which AC current passes. This portion or length of the first
conductor 704 in which current does not pass, and where the
potential is constant, can be referred to as a pigtail. The pigtail
can comprise any length or portion of the first conductor 704.
If the first conductor 704 is coiled, the pigtail can either
comprise an inner portion of the coil towards the center or another
portion of the coil towards the outer radius of the first conductor
704. In an embodiment, the pigtail is used to electrostatically
ignite the plasma, and more than one pigtail can be made from the
first conductor 704.
FIG. 6 illustrates a profile view of an embodiment of a remote
plasma source having conductors arranged in two vertical coils. The
first and second conductors 804, 806 in this embodiment are
solenoids. The description of the fields and function of FIG. 6 is
similar to that described relative to FIGS. 1-4.
But an advantage of the remote plasma source 800 is that
electrostatic coupling drops off faster as a function of distance
from the plasma 842 than inductive coupling. Hence, as each loop of
the first and second conductors 804, 806 are arranged further and
further from the plasma 842, the electrostatic coupling component
is less than the inductive coupling component for each loop. Thus,
the remote plasma source 800 allows a greater percentage of the
power coupled into the plasma 842 to be inductively rather than
electrostatically coupled.
FIG. 7 illustrates a profile view of an embodiment of a remote
plasma source having conductors arranged in a radial and vertical
configuration. The remote plasma source 900 takes advantage of the
increased ratio of inductive to electrostatic coupling made
possible via vertical stacking of the first and second conductors
904, 906 as described with reference to FIG. 6, and the increased
cross section and plasma confinement of the planar disc plasma 942
made possible via radial coiling of the first and second conductors
904, 906 as described with reference to FIG. 4.
Those skilled in the art can readily recognize that numerous
variations and substitutions may be made in the invention, its use,
and its configuration to achieve substantially the same results as
achieved by the embodiments described herein. Accordingly, there is
no intention to limit the invention to the disclosed exemplary
forms. Many variations, modifications, and alternative
constructions fall within the scope and spirit of the disclosed
invention.
* * * * *
References